The present invention relates to a method of orthogonal transform coding/decoding in which transformed coefficients are divided into a plurality of frequency bands, the numbers of coefficients on frequency bands being different from each other, to quantize/de-quantize the coefficients.
FIG. 1 shows a conventional orthogonal transform coding/decoding system which comprises a coding system 10 and a decoding system 17.
In the coding system 10, the data objective to quantization is fed to a front processor 11 and the data thus processed is coded via an orthogonal transformer 12, intermediate processor 13 and a quantizer 14. The data thus quantized is transmitted to the decoding system 17 by a transmitter 15.
More precisely, the data is processed by the front processor 11 with covering a window function. The processed data is transformed by the orthogonal transformer 12 with discrete fourier transformation or discrete cosine transformation.
The frequency band of the transformed data (coefficients) is divided into a plurality of bands by the intermediate processor 13 (sub-band division) The band-divided data is quantized by the quantizer 14.
The number of quantization steps is selected by an additional control processor 16 on the basis of an RMS power (a root-mean-square value), the processor 16 being responsible for other overall adaptive control.
The quantized data and supplementary data are transmitted to the decoding system 17 in the form of a bit stream. The quantized data is decoded via a receiver 18, de-quantizer 19, an intermediate processor 20, inverse orthogonal transformer 21, and a rear processor 22.
The number of de-quantization steps is selected by an additional control processor 23 on the basis of the supplementary data, the processor 23 being responsible for other overall adaptive control.
In such a conventional orthogonal coding/decoding system, effective quantization methods with less distortion of quantized data such as non-uniform quantization, separate quantization in every divided-frequency band and separate quantization in which the number of steps is selected in every frequency band, etc., have been employed.
The above three quantization methods will be explained to start with non-uniform quantization. The probability density function of amplitude of coefficient transformed of an audio or video signal generally exhibits a Laplace or Gauss distribution in which the probability density becomes higher as the amplitude of the coefficient becomes smaller.
In the case of quantization of the transformed coefficients exhibiting those distributions with a constant number of quantization steps, as shown in FIG. 2, a larger number of quantization steps are allotted the region of higher probability density, while a smaller number of steps are allotted for the region of lower probability density to reduce distortion due to quantization (non-uniform quantization). The probability density function and the number of quantization steps are supposed to be an even function and even number respectively in FIG. 2.
An orthogonal coding/decoding system adopting non-uniform quantization is generally designed so as to have the function of minimizing the mean-square error of quantization when the probability density function and the number of quantization steps are supposed
Quantization per divided-frequency band is next explained. Transformed coefficients are divided into a plurality of bands and then quantized with the number of steps required for each divided band. The coefficient values should be normalized with an RMS value (a root-mean-square value) on each band.
For example, suppose that the number of coefficients per band is 16. When the coefficient values are normalized with the RMS value on the band, the normalized coefficient values are in the range of 0.0 to .sqroot.16=4.0. This range is to be the domain of definition.
Accordingly, if quantization is to be conducted per block-length (The number of coefficients is 16.), a quantizer is designed under the condition that, for example, ##EQU1## as the probability density function, where, and the domain of definition is 0.0&lt;x&lt;4.0.
If the number of coefficients per frequency band is constant, an adequate probability density function is set to design quantizing/de-quantizing systems having quantizing threshold level-tables/de-quantized value-tables corresponding to different numbers of quantization/de-quantization steps.
Quantization/de-quantization is performed by comparing coefficient values/quantized values with quantizing threshold levels/de-quantized values on each number of steps.
Quantization by selecting the number of steps per frequency band is explained. FIGS. 3 and 4 show a quantizing system 24 and de-quantizing system 29 respectively in which the number of quantization/de-quantization steps is selected on the basis of an RMS power (an RMS value).
In the quantizing system 24, the number of quantization steps is selected on the basis of the main data including values on each frequency band in a selector 25. The quantization table corresponding to the selected number of steps is selected from a quantization table array 27.
The coefficient values are normalized with the RMS value in a normalizer 26. Then, the normalized coefficient values are compared with quantization threshold levels in the selected quantization table respectively to quantize the normalized coefficient values in a quantizer 28.
The quantized main data and supplementary data including the data required for decoding are supplied to the de-quantizing system 29 from the quantizer 28 and selector 25.
In the de-quantizing system 29, the number of de-quantization steps is selected in a selector 30 on the basis of the supplementary data.
The de-quantization table corresponding to the selected de-quantization steps is selected from a de-quantization table array 31.
The quantized main data is compared with de-quantized values in the selected de-quantization table to de-quantize the quantized main data in a de-quantizer 32.
The de-quantized main data is inverse-normalized with RMS values in an inverse normalizer 33.
Quantization on divided frequency bands with a variable number of coefficients per band is explained. In the case of orthogonal transform and compression coding of audio signals, it is widely known that the audio signals are effectively processed by dividing the critical band known as auditory characteristics into several bands. The domain of definition of the probability density function varies dependent on band.
The following methods have been known as to quantization method adopting quantization tables in the case where the number of coefficients is variable on each band.
(a) Quantization method with a plurality of quantization tables and de-quantization tables designed according to the different numbers of coefficients for respective bands.
(b) Quantization method covering all bands with a quantization table and de-quantization table designed by supposing the mean number of coefficients.
However, (a) is not realistic because the number of quantization tables is increased as the number of bands is increased.
As to (b), quality of quantized data is greatly deteriorated if the number of coefficients varies largely dependent on band.
As shown in FIG. 5A, if the number of coefficients on a band is smaller than a mean value (assumed value), quantization is performed not with all quantization steps but a few steps corresponding to such a small number of coefficients. While if larger than the mean value, this results in lack of quantization steps which causes a large quantization error in the coefficient having a large amplitude on a band.
As understood from the foregoing, the conventional quantization method adopting quantization/de-quantization tables with a variable number of coefficient per band causes problems in construction and quality of the coding/decoding system.